1. Field of Invention
The invention relates, generally, to a method for controlling an internal-combustion engine and, more particularly, to such an engine that is supercharged by a turbocharger.
2. Description of Related Art
Some internal-combustion engines are provided with a turbocharger-supercharging system, which can increase the power developed by the engine, exploiting the enthalpy of exhaust gases for compressing the air aspirated by the engine and, thus, increasing volumetric-intake efficiency.
A turbocharger-supercharging system includes typically a turbocharger provided with a turbine, which is arranged along an exhaust pipe to rotate at a high speed under the bias of the exhaust gases expelled by the engine, and a compressor, which is rotated by the turbine and arranged along the air-feeding pipe to compress the air aspirated by the engine.
In a turbocharger-supercharging system, the operating range of the turbocharger must be kept within a useful zone depending on crank position both for functional reasons (i.e., to avoid irregular or, in any case, low-efficiency operations) and structural reasons (i.e., to avoid the turbocharger from being damaged).
In particular, the useful zone of the operating field is limited by the “surge” line on the left of a “reduced-mass-flow rate/compression ratio” plane and so-called “saturation” line on the right of the same plane. The “surge” line, thus, delimits a first “forbidden” zone and consists of the location of points in which the internal aerodynamic balance of the compressor is disrupted, and a periodical, noisy violent rejection of flow to the mouth occurs, with effects that may destruct the blades.
Patent Application EP1741895A1 describes a method for controlling an internal-combustion engine supercharged by a turbocharger including a compressor, a turbine adapted to rotationally feed the compressor under the bias of the engine-exhaust gases, and a waste-gate valve adapted to adjust the flow of exhaust gases provided into the turbine for controlling the rotation speed of the turbine itself as a function of an objective supercharging pressure required at the compressor outlet. The control method described in Patent Application EP1741895A1 includes the steps of measuring the air pressure taken in at the compressor inlet; determining the mass-flow rate of the compressor; calculating—by a predetermined map that characterizes the operation of the compressor and, as a function of the predetermined rotation-limit speed, measured air pressure and mass-flow rate—a supercharging-limit pressure, which is correlated to the pressure of the air obtainable at the compressor outlet when the turbine rotates at a speed substantially equal to the predetermined limit speed; verifying whether a required supercharging-objective pressure satisfies a predetermined relation with the calculated supercharging-limit pressure; and, if the relation is satisfied, actuating the waste-gate valve for controlling the rotation speed of the turbine according to the supercharging pressure so as to limit the rotation speed of the turbocharger to a value substantially equal to the predetermined limit speed.
Patent Application EP2014894A1 describes instead a method for controlling an internal-combustion engine supercharged by a turbocharger provided with a turbine and compressor that includes establishing on a “reduced-mass-flow rate/compression ratio” plane at least one “operating limit” curve, at least one “intervention” curve of a waste-gate valve that regulates a bypass pipe of the turbine, and at least one “intervention” curve of a Poff valve that regulates the compressor-bypass pipe. The control method according to Patent Application EP2014894A1 includes using the “operating limit” curve for limiting the objective pressure downstream of the compressor used by the engine-control system. The method further provides for controlling the opening of the waste-gate valve if the “intervention” curve of the waste-gate valve is exceeded and Poff valve if the “intervention” curve of the Poff valve is exceeded. Furthermore, the control method of Patent Application EP2014894A1 is effective in ensuring that the operating field of the turbocharger remains within the useful zone under any operating condition of the internal-combustion engine.
The so-called “saturation” line that delimits a second “forbidden” zone and corresponds to the achievement of sonic conditions (and consequent blockage of the flow) at turbine inlet defines the maximum possible flow that the compressor may supply under given conditions of the aspiration environment. Substantially close to the “saturation” line, the turbocharger, thus, reaches very high speeds and can develop the highest power to compress the air aspirated by the engine, thus increasing the volumetric-suction efficiency. Unfortunately, substantially close to the “saturation” line, due to the high speeds at hand, the turbocharger may accelerate in an uncontrolled manner to reach sonic blockage with destructive effects on the turbocharger itself.
Thus, there is a need in the related art for a method for controlling an internal-combustion engine supercharged by a turbocharger. More specifically, there is a need in the related art for such a method that is easy and cost-effective to implement. There is a need in the related art for such a method that is also capable of ensuring that the operating field of the turbocharger remains within the useful zone under any operating condition of the internal-combustion engine. There is a need in the related art for such a method that is also so capable in all cases without reaching sonic blockage.